1. Field of the Invention
The present invention relates to an antenna device comprising an antenna pattern based on an electrode film formed on a dielectric substrate.
2. Description of the Related Art
In order to realize a small size of the antenna device and realize a small size of the communication apparatus, a large number of devices have been hitherto suggested, in which, for example, an antenna pattern based on an electrode film is formed on the surface of a dielectric substrate (see, for example, Japanese Laid-Open Patent Publication Nos. 10-41722, 9-162633, and 10-32413).
Most of the antenna devices can be used by being directly mounted on a circuit board. This fact is an advantage of such antenna devices.
However, the antenna device, which includes the antenna pattern based on the electrode film formed on the surface of the dielectric substrate, involves the following inconvenience. That is, usually, when the device is made compact, then the gain is decreased, and the band is consequently narrowed.
The present invention has been made in view of the points as described above, an object of which is to provide an antenna device which makes it possible to realize a small size while avoiding the decrease in gain and the disadvantage of narrow band.
According to the present invention, there is provided an antenna device comprising an antenna section and a filter section which are formed integrally in a dielectric substrate, wherein the antenna section and the filter section are coupled to one another via a capacitance.
When the antenna section and the filter section are integrated into one unit with the capacitance intervening therebetween, the antenna length is theoretically determined in conformity with the center frequency of the filter section.
The size of the antenna section is dominant as compared with the size of the filter section, in the antenna device in which the antenna section and the filter section are integrated into one unit. Therefore, it is clear from the form or shape thereof that the size of the antenna device greatly depends on the antenna length (wavelength).
Further, it is known for the antenna that the small size causes the decrease in gain and the disadvantage of narrow band.
However, according to the present invention, it has been revealed that the input impedance of the antenna device is not changed even if the antenna length is changed when the antenna device is produced by integrating the antenna section and the filter section into one unit with the capacitance intervening therebetween.
Accordingly, for example, when the antenna length of the antenna section is shortened, it is possible to suppress the decrease in gain to be minimum. The advantage that the input impedance of the antenna device is not changed even when the antenna length is changed results in the successful improvement in yield by adjusting the antenna length during the production step.
It is also preferable for the device constructed as described above that 0.3xc3x97Lrxe2x89xa6Ltxe2x89xa61.2xc3x97Lr is satisfied provided that an antenna length of the antenna section is Lt, and an antenna length measured for a single antenna is Lr.
The reason why the antenna length Lt of the antenna section includes the portion in the range in which it is longer than the antenna length Lr of the single antenna is as follows. That is, although the effect of realization of the compact size is reduced, another effect is obtained such that the margin for mass production is increased when the device is designed, because the change of gain is small even when the antenna length is changed.
The antenna length Lt of the antenna section preferably satisfies 0.6xc3x97Lrxe2x89xa6Ltxe2x89xa61.2xc3x97Lr, and more preferably 0.75xc3x97Lrxe2x89xa6Ltxe2x89xa6Lr.
An antenna for constructing the antenna section may be a monopole antenna, or it may be an antenna having a meander line configuration. Alternatively, the antenna may be an antenna having a helical configuration.
It is also preferable that a length of a resonator disposed on an input side of the filter section is different from a length of a resonator disposed on an output side.
Accordingly, it is possible to counteract the difference in resonance frequency between the respective resonators, which would be otherwise caused by any mismatch between respective impedances on the antenna side and the external circuit side of the filter section. Thus, it is possible to obtain the filter section which has good attenuation characteristics. This results in the high quality of the antenna device.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.